Men should not leave fatherhood too late because children of older fathers are more likely to suffer a raft of psychological and educational problems, scientists have warned.

The biggest study of its kind has found that the children of fathers aged over 45 were at greater risk of autism, bipolar disorder and schizophrenia as well has having lower IQs and poorer academic performance.

Women have traditionally been warned that they should not delay motherhood but because there is no male menopause, men have tended to believe that it is safe to father children in older age.

However when researchers at Indiana University and the Karolinska Institute in Sweden looked at the outcomes of more than two million children born between 1973 and 2001, they discovered worrying trends.

Children born to a 45-year-old man were 13 times more likely to have ADHD than those born to a 24-year-old man and were 3.5 times more likely to have autism and 25 times more likely to have bipolar disorder.

They were also nearly twice a likely to have dropped out of school early and have lower IQs.

“We were shocked by the findings,” said Brian D’Onofrio, lead author and associate professor at Indiana University.

“The specific associations with paternal age were much, much larger than in previous studies. In fact, we found that advancing paternal age was associated with greater risk for several problems such as ADHD, suicide attempts and substance use problems.

“Traditional research suggested advancing paternal age may have diminished the rate at which these problems occur.”

Over the past 40 years the average age of childbearing has been increasing steadily for both men and women.

According to the Office for National Statistics the average woman gives birth just a week short of her 30th birthday while first-time mothers are also older, at just over 28. The average age of fathers is now 32 years and six months.

However many men are still having children into their 50s and 60s. Gordon Brown, the former Prime Minister, had his second son James aged 55, while Sir Paul McCartney fathered his daughter Beatrice aged 61.

Women are born with all their eggs, but male sperm is constantly replicating. Each time sperm replicates there is a chance for a mutation in the DNA to occur. As men age, they are also exposed to numerous environmental toxins which have been shown to cause DNA mutations in sperm.

Molecular genetic studies have shown that the sperm of older men have far more mutations than those of younger men.

Dr Allan Pacey, fertility expert at the University of Sheffield, said men should have children as young as possible.

“This is the biggest and most comprehensive study of its kind and it really highlights that there is a time limit for dads on when they should have their children.

“Men don’t have the menopause, they don’t stop their ability to have children as they age, but what we do see is their partners struggle to get pregnant and have more miscarriages and they increase the risk of their children having a range of problems.

“I think the changes start to take place when a man gets to 40, you start to notice detectable differences in his offspring.

“My advice would be that if you are in a position to have your family early then do it, even if you want to wait a bit longer. Nature wants you to have children early."

Dr D’Onofrio said the study could have important social and public policy implications and employers and the government should make it easier for men and women to have families earlier rather than having to set aside family life in their 20s and 30s for career goals.

“While the findings do not indicate that every child born to an older father will have these problems, they add to a growing body of research indicating that advancing paternal age is associate with an increased risk for serious problems.

“As such, the entire body of research can help inform individuals in their personal and medical decision-making.”

However Dr Mark Pearce, an epidemiologist of Newcastle University, believes the problems could be behavioural as well as biological.

“We have found previously that the children of older fathers are less active so behavioural reasons may also been behind these findings.

“We don’t know the relationship between father and children in this study, and for issues like suicide and substance abuse that is likely to have the biggest impact.

“It’s important to note that the risks are small and not all older fathers are going to have these problems. A lot more research needs to be done before we start changing advice to fathers.”